Actin fibers and microtubules help cilia coordinate the direction and sequence of their beating, Werner et al. show.
At first, the multiple cilia on an embryonic cell are a bit like the musicians in a garage band, each doing its own thing. But over time the filaments turn so that they all beat in the same direction. They also coordinate their timing, so that the cilia at the front of the cell beat first and the ones at the back of the cell stroke last. The planar cell polarity signaling pathway and hydrodynamic forces on the cilia help set up this polarity. Using the cilia-coated epithelial cells of Xenopus embryos, Werner et al. investigated the roles of the actin and microtubule cytoskeletons in the process.
Werner et al. observed actin fibers connecting neighboring cilia. These filaments were absent in the youngest cells that hadn't yet polarized. To determine the fibers' function, the researchers disassembled them using the drug cytochalasin D. Without actin fibers, the cilia couldn't reorient to beat in the same direction. They also couldn't coordinate their timing. However, neighboring clusters of cilia could still synchronize. This local coordination disappeared in cells lacking microtubules, the team found.
These results indicate that two components of the cytoskeleton perform different functions: actin fibers establish cell-wide coordination of cilia orientation and timing, whereas microtubules ensure that cilia are in harmony with their neighbors.